We present CN and CH indices and Ca ii triplet metallicities for 34 giant stars and chemical abundances for 33 elements in 14 giants in the globular cluster M2 .
Assuming the program stars are cluster members , our analysis reveals ( i ) an extreme variation in CN and CH line strengths , ( ii ) a metallicity dispersion with a dominant peak at [ Fe/H ] \approx - 1.7 and smaller peaks at - 1.5 and - 1.0 , ( iii ) star-to-star abundance variations and correlations for the light elements O , Na , Al and Si and ( iv ) a large ( and possibly bimodal ) distribution in the abundances of all elements produced mainly via the s -process in solar system material .
Following Roederer et al .
( 109 ) , we define two groups of stars , “ r + s ” and “ r -only ” , and subtract the average abundances of the latter from the former group to obtain a “ s -process residual ” .
This s -process residual is remarkably similar to that found in M22 and in M4 despite the range in metallicity covered by these three systems .
With recent studies identifying a double subgiant branch in M2 and a dispersion in Sr and Ba abundances , our spectroscopic analysis confirms that this globular cluster has experienced a complex formation history with similarities to M22 , NGC 1851 and \omega Centauri .